The present invention relates to a magnetic disk drive that is capable of properly unloading a head when a head positioning control signal cannot be detected, and to a method for controlling such a magnetic disk drive.
A head load/unload mechanism is used in some conventional magnetic disk drives. If head positioning control cannot be exercised for some reason, such magnetic disk drives perform an unloading position to unload a head onto a ramp.
In the above unloading operation, a constant current for unloading was formerly supplied to a voice coil motor (VCM) to unload the head onto the ramp. In some cases, however, the head bounced off a stopper so that a disk was dented when the head was loaded again onto the disk.
FIG. 5 shows an example of the above head unloading operation. If a constant current is supplied to the VCM to unload the head 12 in a situation where positioning control cannot be exercised while the head 12 is at position A, which is over the disk 10 as indicated in FIG. 5, the head 12 moves over an unloading surface 16 of the ramp 14 and toward the stopper 18. The unloading surface 16 is sloped so that the head 12 needs to move up a slope when the head 12 begins to be unloaded from the disk 10 to the ramp 14. Therefore, the constant unloading current to be supplied to the VCM needs to be large enough for the head 12 to move up the slope. If the head 12 is positioned away from the ramp 14 when positioning control cannot be exercised, a high moving speed results when the head 12 reaches the ramp 14. The reason is that the acceleration time is increased due to electrical current application. Thus, the head 12 moves up the sloped unloading surface 16 at a high speed. Consequently, the speed remains high when the head 12 collides against the stopper 18 as indicated by arrow B in FIG. 5. Therefore, when the head 12, which is put in an unstable attitude due to collision, is loaded onto a landing zone D as indicated by arrow C, the head 12 may collide against the disk 10, thereby denting the disk 10.
To prevent the head 12 from colliding against the disk 10 as described above, a technology disclosed by Japanese Patent Laid-open No. 2001-143228 moves, when the power supply is shut off, the head to the ramp at a low speed by using a small current that is obtained by rectifying a counterelectromotive force, which is generated by a spindle motor, and then causes the head to move up the sloped ramp by supplying a relatively large current from a precharged capacitor. This reduces the speed at which the head collides against the stopper.